Flicker on main form when using PrintDialog

From within my C# app, when I launch a print preview dialog using ShowDialog from a button click on my main form, my main form (behind the preview dialog) flickers whilst the preview job is being built.  The print document is built from the contents of a listbox using the code below.

        private void buttonPrint_Click(object sender, EventArgs e)
        {
                System.Drawing.Printing.PrintDocument pd = new PrintDocument();
                pd.PrintPage += new PrintPageEventHandler(this.pd_PrintPage);
                printPreviewDialog1.Document = pd;
                printPreviewDialog1.ShowDialog();
        }
       
        private void pd_PrintPage(object sender, PrintPageEventArgs ev)
        {
            float linesPerPage = 0;
            float yPos = 0;
            int count = 0;
            float leftMargin = ev.MarginBounds.Left;
            float topMargin = ev.MarginBounds.Top;
            string line = null;

            Font = new Font("Courier New", 9);
            linesPerPage = ev.MarginBounds.Height / Font.GetHeight(ev.Graphics);
            while(count < linesPerPage && nextItem < listBoxResults.Items.Count)
            {
                yPos = topMargin + (count * Font.GetHeight(ev.Graphics));
                line = listBoxResults.Items[nextItem].ToString();
                ev.Graphics.DrawString(line, Font, Brushes.Black, leftMargin, yPos, new StringFormat());
                count++;
                nextItem++;
               
                if (nextItem <= listBoxResults.Items.Count - 1)
                    ev.HasMorePages = true;
                else
                {
                    ev.HasMorePages = false;
                    nextItem = 0;
                    break;
                }
            }
            Font = new Font("Microsoft Sans Serif", 8);             // Set our font back on main form!
        }

How can I stop the flicker on the main form, as it seems the pd_PrintPage event is being fired by the ShowDialog() method itself?